1. Field of the Invention
The invention relates to organic light emissive devices, to methods of making such devices and the use of cathodes therein.
2. Related Technology
Organic light emissive devices (OLEDs) generally comprise a cathode, an anode and an organic light emissive region between the cathode and the anode. Light emissive organic materials may comprise small molecular materials such as described in U.S. Pat. No. 4,539,507 or polymeric materials such as those described in PCT/WO90/13148. The cathode injects electrons into the light emissive region and the anode injects holes. The electrons and holes combine to generate photons.
FIG. 1 shows a typical cross-sectional structure of an OLED. The OLED is typically fabricated on a glass or plastics substrate 1 coated with a transparent anode 2 such as an indium-tin-oxide (ITO) layer. The ITO coated substrate is covered with at least a layer of a thin film of an electroluminescent organic material 3 and cathode material 4 of low workfunction metal such as calcium is applied, optionally with a capping layer of aluminum (not shown). Other layers may be added to the device, for example to improve charge transport between the electrodes and the electroluminescent material.
There has been a growing interest in the use of OLEDs in display applications because of their potential advantages over conventional displays. OLEDs have relatively low operating voltage and power consumption and can be easily processed to produce large area displays. On a practical level, there is a need to produce OLEDs which are bright and operate efficiently but which are also reliable to produce and stable in use.
The structure of the cathode in OLEDs is one aspect under consideration in this art. In the case of a monochrome OLED, the cathode may be selected for optimal performance with the single electroluminescent organic material. However, a full colour OLED comprises red, green and blue light organic emissive materials. Such a device requires a cathode capable of injecting electrons into all three emissive materials, i.e. a “common electrode”.
It is known that a layer of metal fluoride located between the organic emissive layer (or organic electron transporting layer, if present) and the metal cathode can result in an improvement in device efficiency—see for example Appl. Phys. Lett. 70, 152, 1997. This improvement is believed to result from a reduction in the barrier height at the polymer/cathode interface, allowing improved electron injection into the organic layer(s). A mechanism of device degradation using the LiF/Al cathode is proposed in Appl. Phys. Lett. 79(5), 563-565, 2001 wherein LiF and Al may react to release Li atoms that can migrate into the electroluminescent layer and dope the electroluminescent material. However, the present inventors have found the LiF/Al cathode to be relatively stable, its main drawback being relatively low efficiency (in particular when used as a common cathode). A more efficient arrangement utilises a bilayer of calcium and aluminum over LiF in place of aluminum alone, which is described as a common cathode in Synth. Metals 2000, 111-112, p. 125-128, however this is a less stable system due to greater liberation of metal as compared to the LiF/Al cathode. In particular, it is reported in WO 03/019696 that degradation is particularly marked for devices comprising this cathode and electroluminescent materials comprising sulfur such as the red emitting polymer comprising the trimer repeat unit thiophene-benzothiadiazole-thiophene.
According to WO00/35028 WO 00/35028, a light emissive device is provided which includes a light absorbent layer comprising graphite and/or a fluoride or oxide of a low work function metal such as lithium. According to this arrangement, the cathode may be formed of lithium fluoride optionally codeposited with aluminum for use as an electron-injecting layer which is light absorbent. Aluminum has a relatively high workfunction. U.S. Pat. No. 6,278,236 also provides a multilayer organic electroluminescent device with an electron-injecting layer. In this arrangement the electron-injecting layer includes aluminum and at least one alkali metal halide or at least one alkaline earth metal halide. A composite electron injection layer comprising lithium fluoride and aluminum is exemplified. Other high work function cathode materials such as gold are suggested. Another composite cathode is described in Jabbour et al in Applied Physics Letters 73 (9), 1185-1187 (1998). Aiming to avoid the use of low work function metals to avoid unreliability, a composite cathode is described comprising lithium fluoride or caesium fluoride and aluminum. US 2001/0051284A also describes a composite electron injection layer in a multilayer organic electroluminescence device. The electron-injection layer preferably comprises low work function metal oxides or metal halides such as those of lithium, magnesium and yttrium. Metals having a relatively high work function are mixed in the electron-injection layer, preferably aluminum, indium, silver or gold, each of which has a work function in excess of 4.2 eV. Indium and aluminum are the metals disclosed in the Examples.